A liquid crystal display device is classified, on the basis of an operation mode of liquid crystal, into PC (phase change), TN (twisted nematic), STN (super twisted nematic), BTN (bistable twisted nematic), ECB (electrically controlled birefringence), OCB (optically compensated bend), IPS (in-plane switching) and VA (vertical alignment) devices, and the like. Then, the liquid crystal display device is classified, on the basis of a drive system of device, into PM (passive matrix) and AM (active matrix) devices. The PM (passive matrix) device is classified into static and multiplex devices, and the like; and the AM device is classified into TFT (thin film transistor) and MIM (metal insulator metal) devices, and the like.
These devices contain a liquid crystal composition. In order to improve characteristics of the device, it is preferable that this composition has adequate physical properties. General physical properties which a compound as a component of the composition is required to have are as follows.    (1) Chemical stability and physical stability.    (2) High clearing point. The clearing point is a transition temperature between a liquid crystal phase and an isotropic phase.    (3) Low lower limit temperature of a liquid crystal phase. The liquid crystal phase as referred to herein means a nematic phase, a smectic phase, or the like.    (4) Low viscosity.    (5) Adequate optical anisotropy.    (6) Adequate dielectric anisotropy. In many cases, a compound having large dielectric anisotropy has a high viscosity.    (7) High resistivity.
A liquid crystal composition is prepared by mixing a lot of liquid crystalline compounds. Accordingly, it is preferable that such a liquid crystalline compound is well miscible with other compounds. Since a device may possibly be used at a below-freezing temperature, compounds having well compatibility at low temperatures are preferable. A compound having a high clearing point or a low lower limit temperature of a liquid crystal phase contributes to the wide temperature range of a nematic phase in the composition. A preferred composition has a low viscosity and optical anisotropy suitable for the mode of a device. Large dielectric anisotropy of a compound contributes to a low threshold voltage of a composition. By using such a composition, it is possible to obtain a device having characteristics such that the useful temperature range is broad; the response time is short; the contrast ratio is large; the drive voltage is small; the electric power consumption is low; and the voltage retention is high.
Hitherto, there have been scarcely known examples in which a dihydrocoumarin derivative is applied as a liquid crystal display device. The following group of compounds having a structure analogous to a liquid crystal has been known up to date. However, in all of these compounds, their applications are not relevant to a liquid crystal display device, and characteristics, physical properties, and the like of these compounds as a liquid crystal are not clarified at all.
Oriental Journal of Chemistry (1987), 3(2), 174-8
Journal of Organometallic Chemistry (1990), 387(3), 381-90